1. Field of the Invention
The present invention relates to an amplifying circuit, and more particularly to a power amplifying circuit for increasing linearity.
2. Description of the Prior Art
Power amplifiers have been widely used in different kinds of wired or wireless system applications. Normally, there are two means of determining the effectiveness of a power amplifier, the first one being the power gain and the second being the linearity of the power amplifier. The linearity can determine the distortion between the input signal and the output signal of the power amplifier. Furthermore, in advanced CMOS manufacturing techniques, utilizing the ideal system-on-chip (SOC) to implement the whole wireless transmitter chip is more popular. Please refer to C. Wang, M. Vaidyanathan, and L. E. Larson, “A Capacitance Compensation Technique for Improved Linearity in CMOS Class-AB Power amplifiers,” IEEE-J. Solid-State-Circuits, vol. 39, no. 11, pp. 1927-1937, November 2004 for more information. According to the prior art, the nonlinearity of the CMOS power amplifier is mainly caused by two reasons: the first one is the transconductance Gm of the transistor, which determines the linearity of the transistor; and the second reason is the nonlinear capacitor Cgate at the gate of the transistor, which determines the linearity of the output of the previous stage circuit. The above-mentioned prior art has disclosed a capacitive compensation method to improve the nonlinear capacitor Cgate at the gate of the transistor. However, this conventional method can only be utilized in a CMOS power amplifier having a class AB configuration. Furthermore, the prior art requires an increased chip area and the power gain is not ideal. For the OFDM system, which requires a very high linearity (e.g. the linearity requirement of the specification of 802.11g is as high as 25 dBm at P1 dB), only a power amplifier implemented by a class A configuration can be adopted. However, a power amplifier with a class A configuration will have a bad linearity caused by the above-mentioned problem.